The administration of the iron- and gallium-chelating agent, desferrioxamine (DFO), to mice after the injection of 67Ga has been shown to improve tumour to normal tissue ratios. Time and dose parameters have been studied. The optimal time was about 7 h after 67Ga, when the tumour to blood ratio was increased 16 times compared with controls. The effect was due to the greater retention of 67Ga in tumour compared with other tissue. Tumour retention after DFO was 78% compared with controls at 7 h, giving a tumour to blood ratio of 25.9. Tumour retention of 67Ga correlated well with the proportion of cell-bound activity which was localised in the lysosomes. Prior administration of iron increased lysosomal accumulation of 67Ga in tumour cells, and hence the retention of the nuclide after the use of DFO.